1. Field of the Invention
The present invention relates to an image reader for reading image data optically from an original to be read by using an image pick-up device such as a CCD (Charge Coupled device) which reads images in a fast scanning direction and by moving a scanning carriage with a mirror, etc. mounted thereon in a slow scanning direction with use of a stepping motor for example.
2. Description of the Prior Art
Generally, in an image reader, if the data processing performance of a system connected at a succeeding stage to the image reader is sufficiently high in image data reading speed, it is possible to effect image reading without stopping the scanning operation, but if the data processing performance (also including the data transfer band width) of the succeeding system is low, it is necessary to synchronize the image data reading speed with the lowest processing speed portion. For example, in the case where a personal computer (hereinafter referred to as xe2x80x9cPCxe2x80x9d) is connected as the succeeding system to the image reader and images are to be inputted to the PC, the data transfer speed at I/F to the PC and the data writing speed of an external storage become a bottleneck. For properly controlling the image data reading speed there has been proposed, for example, such an image reader as disclosed in Japanese Published Unexamined Patent Application No. Hei 9-200433, in which the procedure of image data reading (image data buffering)xe2x86x92stop readingxe2x86x92transfer of image dataxe2x86x92restart readingxe2x86x92stop readingxe2x86x92transfer of image dataxe2x86x92 . . . is repeated and image data is read intermittently.
Such intermittent data reading is adopted in many image readers because the data processing performance of the succeeding system is undefined in many cases.
For realizing a satisfactory image quality of read image data in such an intermittent image reader it is necessary to prevent the occurrence of an image quality trouble such as image displacement at a joint portion of intermittent operations. To meet this requirement, in a conventional image reader which performs an intermittent image data reading, as shown in FIG. 5, there is adopted a line step method in which a fast scanning synchronizing signal (LineSync signal) for controlling the read timing in the fast scanning direction and a sensor drive signal (CCDCLK signal) for driving a CCD, etc. (both signals will hereinafter be referred to together as xe2x80x9cLSxe2x80x9d) as well as a motor drive control signal (MotorClock signal, xe2x80x9cMTCLKxe2x80x9d hereinafter) for controlling the operation of a motor in the slow scanning direction, are synchronized their phases and frequencies and an LS cycle is made into an integer multiple of an MTCLK cycle. By adopting such a line step method it is made possible to effect joint alignment in the intermittent operation.
Although the above line step method is easily applicable to a scanner for a PC and a facsimile device in which the motor driving speed (scanning speed in the slow scanning direction) is relatively low and there is a high degree of freedom, it is difficult to apply the same method to a copying machine or the like in which the motor driving speed is relatively high and the degree of freedom is low.
This is because in a copying machine or the like the driving speed of a scanning carriage must basically be made equal to the processing speed of an image output terminal (IOT) and also because it is necessary to attain optimization of vibration, thus making it difficult to synchronize the reduction ratio from a drive motor to the scanning carriage with the LS cycle. A further reason is that in a copying machine or the like constructed so as to change the motor speed and perform scaling processing when an arbitrary scale factor has been set in the slow scanning direction it is impossible to keep constant the relation between phase and frequency of LS and MTCLK.
In an image reader used in such a copying machine or the like, if an intermittent drive control is made without adopting the line step method and without taking any countermeasure, there will occur a phase shift between MTCLK (=a physical position of the scanning carriage) and LS, as shown in FIG. 6, thus giving rise to a fear that there may occur a read position displacement corresponding to a maximum of one line before and after an intermittent operation. As a result, an oblique line displacement or the like will occur at a joint portion of read images. Moreover, in reading color images, there will occur such an image quality trouble as color misregistration.
On the other hand, it may be effective to constitute an image reader in such a manner that after reading image data, not intermittently, and once storing the read data in a memory such as a page memory, there are performed read-out of image data from the memory in accordance with the data processing performance of a succeeding system and transfer of the data to the succeeding system. In this case, however, a large capacity of memory is needed because the quantity of image data has been increasing in an accelerative manner with the recent tendency to higher read resolution and coloring of image, thus leading to an increase of the device cost.
The present invention has been accomplished in view of the above-mentioned circumstances and provides an image reader which, in performing an intermittent image data reading, can prevent the occurrence of an image trouble such as image displacement at a joint portion of intermittent operations without the need of such a memory as a page memory even if a signal LS for making a read control in the fast scanning direction and a signal MTCLK for controlling the movement in the slow scanning direction are asynchronous with each other.
The image reader of the present invention is provided with an image pick-up unit which reads an image in a fast scanning direction in accordance with a fast scanning synchronizing signal of a predetermined cycle, a drive unit which causes the image pick-up unit to move in the slow scanning direction in accordance with a drive control signal asynchronous with the fast scanning synchronizing signal, an intermittent read instruction unit which generates a stop instruction and a restart instruction for allowing the image reading to be performed in the slow scanning direction by the image pick-up unit, a phase holding unit which recognizes and holds a phase difference between the fast scanning synchronizing signal and the drive control signal upon issuance of a stop instruction from the intermittent read instruction unit, and a phase correcting unit which, upon issuance of a restart instruction from the intermittent read instruction unit, corrects a phase difference between the fast scanning synchronizing signal and the drive control signal on the basis of the phase difference recognized and held by the phase holding unit.
In the image reader of the above configuration, the drive unit makes a switch-over whether the image pick-up unit is to be moved in the slow scanning direction or not in accordance with the stop instruction and the restart instruction both generated by the intermittent read instruction unit, whereby the image pick-up unit reads images in the slow scanning direction intermittently. In this image reader, in accordance with the stop instruction and the restart instruction from the intermittent read instruction unit the phase holding unit holds a phase difference between a fast scanning synchronizing signal (say, LS) and a drive control signal (say, MTCLK), and on the basis of the phase difference thus held the phase correcting unit corrects the phase difference between the fast scanning synchronizing signal and the drive control signal. As a result, the phase difference between both signals at the time of restart of the intermittent read becomes equal to that at the time of stop of the intermittent read. Thus, in this image reader, even if the fast scanning synchronizing signal and the drive control signal are asynchronous, the occurrence of a phase shift between both signals at the time of stop and restart of the intermittent read is prevented.